Adjacent transponders contained in satellites often use different polarizations. In many applications, vertical polarization is used on odd numbered transponders and horizontal polarization is used on even numbered transponders. Many satellites also use either right-hand or left-hand circular polarization for signal discrimination. By polarizing the signals of the satellite, the same frequencies may be reused. This is particularly important since signal traffic on satellites is increasing. One problem with reusing frequencies to increase the capacity of a satellite is that utilizing dual-polarization at K-band and higher frequencies is often plagued by attenuation and depolarization due to the transmission of the signals through rain or other atmospheric conditions.
One method for compensating for cross polarization is by increasing the effective isotropic radiated power (EIRP). By increasing the EIRP, the signal to noise ratio of the signal is increased. The approach would transmit higher EIRP on both channels. However, during a heavy rain event, the self interference caused by cross polarization becomes the limiting noise source. Increasing EIRP in one channel increases interference in the other channel. This solution has several drawbacks including high power consumption at both the receiving end and the transmitting end and that it work.
One method of canceling for cross polarization interference involves injecting pilot tones into one polarized wideband channel at the transmitter. The pilot tones are used to compute an estimate of the magnitude and phase of the interfering signal at the receiver. The phases and the amplitudes of the leakage tones are measured and used to compute the phase and amplitude at the mid-band point of the wide band channel. The computed quantity is then used to control a complex weighting circuit.
One problem with the pilot tone method for removing interference is that the method is only capable of removing average cross polarization interference across the transmitted bandwidth. The pilot tones are located at either side of the transmitted frequency band to avoid adding interference themselves. Thus, it is assumed that the dispersion is linear between the two pilot tones. Studies have shown, however, that cross polarization interference is frequency dependent and may vary widely across a frequency band. This can reduce the accuracy of the calculation.
The pilot tone method known for reducing cross polarization interference is only a "one-sided" device. That is, unless a second device is built and a second set of pilot tones is inserted into the primary channel, cancellation occurs only in the primary channel. The secondary channel, which is for the pilot tones, is used only to provide interference measurements to the primary channel. Thus, two independent devices are required and two sets of pilot tones must be injected, one in each channel
Other possible techniques for cross polarization interference cancellation rely on techniques similar to base band equalization. Such systems require demodulation of the received signal. Because the systems require demodulation, the system is not flexible to changes in data rate, modulation format, and the number of carriers.
What is needed is a method for integrating a cross polarization interference canceller that is independent of waveform, data rate, and the number of carriers on the system. Further, such a method would not have any additional reference data needs, such as pilot tones.